Sintered R-T-B based magnets (where R is at least one of the rare-earth elements, T is Fe with or without Co, and B is boron) are currently used extensively in rotating motors, linear motors, voice coil motors (VCMs) and various other rotating machines. In this description, the “rare-earth elements” refer to a total of 17 elements consisting of Sc (scandium), Y (yttrium) and lanthanoids.
Sintered R-T-B based magnets certainly have great remanence but their relative Curie temperature is so low that irreversible flux loss will occur easily, which is one of the old drawbacks of the sintered R-T-B based magnets.
If a sintered R-T-B based magnet is used in a motor, that magnet will not only be exposed to a great demagnetization field but also come to have its temperature raised by the heat generated by a coil. That is why to prevent the sintered R-T-B based magnet from causing such irreversible flux loss, the coercivity thereof should be increased.
According to conventional technologies, at least one of Dy and Tb, which are heavy rare-earth elements RH, is added a lot to a sintered R-T-B based magnet in order to minimize such irreversible flux loss. If a lot of heavy rare-earth element RH is added, however, the coercivity will certainly increase but the remanence will rather decrease, which is a problem. The reason is that if the heavy rare-earth element RH is added, then Nd or Pr that will produce high remanence will be replaced as its R component in an R2T14B compound, which is the main phase of the sintered R-T-B based magnet, with Dy or Tb that will produce only low remanence.
On top of that, since Dy and Tb are very rare and expensive elements, it is not a cost-effective measure, either, to add a lot of Dy or Tb.
Thus, to overcome such problems, various techniques for increasing the coercivity with the amount of the heavy rare-earth element RH added minimized have been proposed so far. For example, it was proposed that the heavy rare-earth element RH be added in a high concentration only to a shell portion of a main phase crystal grain, where the local anti-magnetic field has so great strength as to start magnetization reversal. And a two-alloy process was tentatively used as a specific method to take for that purpose.
Specifically, according to the technique disclosed in Patent Document No. 1, two different kinds of R-T-B based alloy powders are mixed together. In this case, those two alloy powders may have the same R mole fraction and the other main components thereof may also have the same composition except the mole fractions of Dy, Nd and other R elements only. Or those two alloy powders may have the same R mole fraction and the other main components thereof may also have the same composition except the mole fractions of Dy and Nd and other R elements and Fe that has been partially replaced with a refractory metal such as Nb. In this manner, an R-T-B based sintered permanent magnet, of which the main phase crystal grains have a characteristic Dy concentration distribution and which has a main phase crystal grain size distribution that contributes to achieving high Br and high (BH)max, can be obtained with good stability.
Patent Document No. 2 discloses a technique for making a sintered R-T-B based magnet in which three R2T14B phases including a heavy rare-earth element RH in high, low and intermediate concentrations, respectively, are present in mixture in a single crystal grain by providing two R2T14B based alloys including, as rare-earth elements R, light and heavy rare-earth elements RL and RH in mutually different ratios, mixing those two alloys together, pulverizing the mixture, and then sintering the pulverized powder.
Patent Document No. 3 discloses a technique for making a sintered rare-earth magnet by mixing together a first component powder mainly composed of an intermetallic Nd2Fe14B compound and a second component powder mainly composed of R(Cu1-xTx) and/or R(Cu1-xTx)2, compacting the mixture under a magnetic field, and then subjecting the compact to liquid crystal phase sintering.
Patent Document No. 4 discloses a technique for producing a rare-earth magnet by performing the steps of: mixing first and second magnetic powders together to obtain a mixed magnetic powder; compacting the mixed magnetic powder to obtain a green compact; and sintering the green compact. In this case, the first magnetic powder is made of a magnetic material including rare-earth elements, transition elements and boron (B), has a mean particle size of 10 μm or less, and includes Dy as one of the rare-earth elements. On the other hand, the second magnetic powder is made a magnetic material including rare-earth elements, transition elements and boron (B), has a second mean particle size that is also 10 μm or less but that is different from that of the first magnetic powder, and includes Dy in a second mole fraction that is different from the Dy mole fraction of the first magnetic powder.
And Patent Document No. 5 discloses a technique for making a sintered R-T-B based magnet, where the main phase crystal grains have a core-shell structure, which consists of a core portion and a shell portion that surrounds the core portion and in which the concentration of a heavy rare-earth element is lower by at least 10% in the core portion than in the surface region of the shell portion. In such a sintered R-T-B based magnet, the average of an L/r ratio, which is the ratio of the shortest distance L from the surface of the shell portion of a main phase crystal grain to its core portion to the equivalent circle diameter r of the main phase crystal grain 1, falls within the range of 0.03 to 0.40.
Citation List
Patent Literature
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